1. Field of the Invention
The present invention relates to a method and system for correcting magnetic resonance images for animation display.
2. Description of the Related Art
A magnetic resonance imaging (MRI) system is of such a type that, by applying a uniform static magnetic field to a predetermined position of a subject and applying a radio frequency (RF) magnetic field by an RF coil in a direction to that static magnetic field, an MR phenomenon occurs at a predetermined slice of the subject and, after the application of the RF magnetic field has been completed, an MR signal which originates from the subject is detected by the RF coil. Upon the superimposition of a liner-gradient magnetic field onto the static magnetic field the MR signal is obtained which is Fourier-transformed to acquire projection data. A computed tomogram image corresponding to the slice is reconstituted by projection data.
In this type of MRI system, in synchronization with the heart rate of the subject an MR signal at each time phase is reconstituted for continuous display (animation display).
FIG. 1 shows the sequence for obtaining an MR image at each time phase which corresponds to a region of interest (ROI). In FIG. 1, TR represents a repetition time period and TR' initial repetition time period within one heart rate. Here the image value I of the MR image is given by: EQU I=A.exp(-TE/T2)(1-exp(-TR/T1))
where
A: the density of protons; PA1 TE: the echo time; PA1 T1: the longitudinal relaxation time; and PA1 T2: the traverse relaxation time.
If, in this case, the initial repetition time period TR' is different from the other repetition time period TR, then a shift occurs at an image value I.
If an MR image is to be obtained with respect to each time phase corresponding to a different ROI, MR data on positions 1, 2, 3 and 4 are collected at a first scan; MR data on positions 2, 3, 4 and 1 at the second scan; MR data on positions 3, 4, 1 and 2 at the third scan and MR data on positions 4, 1, 2 and 3 at the fourth scan, respectively. Since, in this case, a multi-slice method is employed in changing the positions on the ROI of the subject, an image value varies in accordance with the time phase and hence display images are very difficult to view as an animation display.
A growing demand has been made for an MRI system which can readily display an MR image, in an animation fashion, which is obtained for each time phase in synchronization with the heart rate of a subject.